The invention relates to the general field of integrated circuit packaging with particular reference to the absorption of stress at the contacts.
In the macro world (measured in fractions of an inch) spring loaded contacts are widely used because of their ability to absorb stresses associated with pressing two surfaces together. This is of particular importance if either, or both, surface is fragile and subject to damage if the contact force were to be transmitted to it. Contacting systems of this type are commonly formed by seating a ball in a cylindrical cavity, behind an opening that is slightly smaller than the diameter of the ball, and mounting a coil spring between the ball and the other end of the cylinder.
Similar stresses are associated with contacting systems that are used in the micro world (measured in microns and fractions thereof) but, to the best of our knowledge, spring loaded contacts similar to those described above have not yet been developed. A particular example of a micro system in which a fragile set of surfaces need to make and maintain good contact with a second set of surfaces, is a packaged integrated circuit where contact pads that are part of a chip need to make contact with solder balls located on the outside of the package. These solder balls will be used later to attach the package to the next packaging level (for example a printed circuit board) and it is important that the associated thermal and mechanical stresses not be transmitted down to the chip level.
The reason that spring loaded contacts have thus far not been used in the micro world is due, we believe, to the difficulty of fabricating a truly three dimensional object such as a coil spring (as opposed to a multi-layer structure with vertical connections between layers).
The closest prior art to (though still substantially different from) the present invention that we have found are a pair of patents by Little (U.S. Pat. No. 5,663,596xe2x80x94structure) and (U.S. Pat. No. 5,665,648xe2x80x94method). In Little""s invention, chip contacting pads are located on the vertical edges of the integrated circuit chip. The latter is attached to a substrate through spring contacts that have a base portion joined to a line on the substrate and a spring portion that presses against the edge of the chip. To fabricate the spring, a release layer is first laid down. Then a metal film is deposited under conditions known to induce high tensile stress so that, when the release layer is etched away, the freed metal curls up to a sufficient degree to allow the chip to be pushed against its underside, thereby forming a spring contact.
While Little""s structure is not a coil, coil-like structures have been described in the prior art for use as micro inductors. These structures are, however, not truly three dimensional and comprise multiple layers of open rings that are then connected in series by means of vertical vias. An example of this is described by Lee (U.S. Pat. No. 5,831,331). Such structures, while working as inductors, could not be used to provide stress relief as each ring is separated from its neighbors by a rigid layer. Even if the latter was made to be flexible, absorption of stress would be limited to the vias which have very little flexibility so that any, except very small, stresses would be transmitted between contacting surfaces.
It has been an object of the present invention to provide a structure capable of absorbing stress between an integrated circuits package and the semiconductor.
Another object of the invention has been to provide a process for manufacturing said structure.
A still further object of the invention has been that said process be fully compatible with existing manufacturing techniques for integrated circuits.
These objects have been achieved by introducing a micro-spring between the solder ball and the contact tab. The process for manufacturing such a structure begins with a fully completed integrated circuit on whose surface freestanding metal posts are formed, each post being in contact with an I/O tab. Using a leveling plate at elevated temperature, the posts are given a permanent tilt relative to the surface and are then encapsulated in an elastomer. This subprocess may then be repeated as many times as desired with the direction in which the posts lean being changed by 90 degrees at each iteration. This results in the formation of an orthogonal spiral which acts as a coil spring that absorbs stress originating at the solder ball.